Get him a Surefire 6P light with a pigtail switch, a spare P60 lightbulb and a dozen Surefire batteries. You can get those on the EE or on eBay.
Other things you can get him is a portable DVD Player for his time off in between missions; also get him a portable video game (PSP, Game Boy DS or a Game Boy Advanced) plus a couple games.
As for gear, he'll probably get plenty of gear through RFI, but a pair of Oakley M-Frames (flat black with grey lenses) will be good as well.
One thing he can really use over there are basic toiletries, like toothpaste, baby wipes (they are worth gold), sunscreen; edibles like hard candy, beef jerky, powdered juice drinks like Gatorade; plenty of magazines (Stuff, Maxim, FHM, Men's Helth, GUns & Ammo, Soldier Of Fortune, Super Chevy, etc...) and plenty of letters of support and love from friends, relatives, church groups, schools, etc...
My prayers are with your son and with all his fellow Marines. THan you for your son's service and hope for a safe return home.
SEMPER FI!!!